Rotary position sensing is used in a number of applications, such as motor position feedback control and/or commutation, cam and crank shaft position sensing for controlling ignition timing, misfire detection, engine speed monitoring etc, robotics, machine tool position control and the like.
Optical encoders, resolvers, reluctance sensors, hall effect sensors and potentiometers are among the most common sensors used for rotary position sensing. Each has its own particular benefits, but no technology offers a truly universal solution. As a result, design engineers require knowledge of a number of sensing technologies and interfaces to solve position sensing problems.
The applicant has already proposed a rotary position sensor in EP 0760087. The system described in this patent has a fixed sensor element, manufactured using printed circuit board (PCB) technology, which carries a number of conductors constructed as a set of quadrature sensor coils and an excitation coil. In operation, an AC current is applied to the excitation coil which generates a uniform AC magnetic field over the sensing area of interest. This magnetic field couples with a wireless, contactless moving electrical PCB resonator comprising a coil and capacitor. In response, the resonator creates its own local oscillating magnetic field at the same AC frequency, which couples back into the sensor coils. The EMFs generated in the sensor coils are then used to determine the resonator position relative to the fixed sensor element.
The pattern of conductors on the sensor element which form the sensor coils are periodic, with three periods extending around the circuit board, thereby allowing absolute position sensing over an angle of 120xc2x0. In addition, the coil which forms part of the resonator has three lobes which are symmetrically spaced around the resonator PCB to match the periods of the sensor coils. As a result, both the sensor coils and the resonator coil have rotational symmetry and the system is therefore, relatively immune to effects of unwanted tilt or misalignment between the sensor PCB and the resonator PCB. A full absolute 360xc2x0 sensor could be made using the teaching of EP 0760087. However, such a position sensor would only be suited to relatively low accuracy 360xc2x0 measurements because (i) unwanted spatial harmonics and electronic processing artefacts usually limit the accuracy of the device to approximately 0.2% of a period, i.e. xc2x10.7xc2x0 in this case; and (ii) the sensor would be more sensitive to offsets and tilts of the resonator PCB relative to the sensor PCB because of the lack of rotational symmetry which would result since the resonator coil would only have a single lobe to match the single period of the sensor coils.
The present invention therefore aims to provide a more accurate rotary position sensor which is less sensitive to PCB misalignment and tilt.
According to one aspect, the present invention provides a rotary position detector comprising first and second relatively rotatable members, the first member comprising (a) a first field generator for generating a field which spatially varies in a cyclic manner with an angular frequency of M; and (b) a second field generator comprising a winding which is wound so as to have a plurality of loop portions which are circumferentially spaced apart, for generating a field which spatially varies in a cyclic manner and which comprises a fundamental component with an angular frequency of N and some harmonics thereof, wherein N is less than M and wherein the circumferential spacing between the loop portions is arranged so as to reduce the amplitude of predetermined ones of the harmonic components generated by the second field generator; said second member comprising (a) a first sensor which is sensitive to fields which spatially vary in a cyclic manner with an angular frequency of M; and (b) a second sensor which is sensitive to fields which vary in a cyclic manner and with an angular frequency of N; whereby, in response to a magnetic field being generated by the first and second field generators, first and second signals are generated by the first and second sensors respectively, which signals vary with the relative angular position of the first and second members; and means for determining the relative angular position of the first and second members from the first and second signals.
Such a rotary position detector is advantageous because it is less sensitive to misalignments between the magnetic field generators and the sensors because the loop portions of the second field generator have been arranged so as to reduce predetermined ones of the harmonic components. The components which are reduced are preferably those with an angular frequency of Mxc2x11, since it is these components which provide the maximum distortion to the position measurement signals when there is a misalignment.
The present invention also provides a transducer for use in such a rotary position detector which comprises the field generators and the sensors discussed above. These field generators and sensors are preferably formed as conductive tracks formed on two or more layers of printed circuit board, since these are relatively easy and cheap to manufacture.